Ho Sun Lim

Multifunctional Hybrid Fabrics with Thermally Stable Superhydrophobicity

By controlling both surface geometry and chemistry, lotus leaves have an interesting capacity to cleanly maintain their obverses, even in a dirty pond. Since lotus leaves strongly resist the sticking of water due to the presence of a hydrophobic waxy layer covering their microor nanostructured surfaces, contaminants are easily removed as water droplets roll off on the leaf surfaces. Inspired by their unique characteristics, superhydrophobic surfaces have attracted considerable attention for numerous industrial, commercial, and scientific applications. Until now, many artificial surfaces with extreme water repellency have been developed by means of complicated microand nanofabrication strategies, including photolithography, electrodeposition, plasma etching, colloidal assembly, chemical vapor deposition, and templating. However, although they enable a profound understanding of abnormal wetting behavior, the disadvantages of these surfaces, such as their high cost, time-consuming preparation, and poor throughput, restricted their practical application. On the other hand, electrospinning technology, as a cost-effective and simple fabrication method, provides a promising tool for creating microand nanostructures over large areas. Electrospinning is a fascinating option for the production of superhydrophobic fabrics, resulting from the generation of continuous nanofibers from solutions or melts of a variety of materials under high electric fields. A number of hybrid materials can be electrospun into fibers with diameters ranging from a few nanometers to micrometers, depending on the viscosity and concentration of the source solution, the molecular weight of hybrimers, and applied voltages. Themorphologies of thesematerials can be widely and facilely controlled, yielding twoor three-dimensional nanofiber assemblies on any substrate. Interestingly, the mechanical properties of fibrous sheets are substantially improved when fiber diameters are reduced to the hundred-nanometer scale. Fibers of such size allow the fabrication of freestanding nanofiber

Robust superhydrophobic mats based on electrospun crystalline nanofibers combined with a silane precursor.

We demonstrate the fabrication of solvent-resistant, mechanically robust, superhydrophobic nanofibrous mats by electrospinning of poly(vinylidene fluoride) (PVDF) in the presence of inorganic silane materials. The solvent resistance and mechanical strength of nanofibrous mats were dramatically increased through the crystallization of as-spun PVDF fibers or incorporation of a tetraethyl orthosilicate (TEOS) sol into the nanofibrous matrix. The electrospun nanofibrous mats yielded a water contact angle of 156 degrees that did not vary with TEOS content. The solvent resistance and mechanical robustness of the electrospun mats were significantly enhanced through extensive cross-linking of TEOS, even after short PVDF annealing times. The interpenetrating polymer network, which embeds polymer chains in a TEOS network, allows the fabrication of robust functional nanofibers by combining semicrystalline polymers with electrospinning techniques.

Polyelectrolyte Interlayer for Ultra-Sensitive Organic Transistor Humidity Sensors

We demonstrate low-voltage, flexible, transparent pentacene humidity sensors with ultrahigh sensitivity, good reliability, and fast response/recovery behavior. The excellent performances of these devices are derived from an inserted polyelectrolyte (poly[2-(methacryloyloxy)ethyltrimethylammonium chloride-co-3-(trimethoxysilyl)propyl methacrylate] (poly(METAC-co-TSPM)) interlayer, which releases free Cl- ions in the electrolyte dielectric layer under humid conditions and boosts the electrical current in the transistor channel. This has led to extreme device sensitivity, such that electrical signal variations exceeding 7 orders of magnitude have been achieved in response to a 15% change in the relative humidity level. The new sensors exhibit a fast responsivity and a stable performance toward changes in humidity levels. Furthermore, the humidity sensors, mounted on flexible substrates, provided low voltage (<5 V) operation while preserving the unique ultrasensitivity and fast responsivity of these devices. We believe that the strategy of utilizing the enhanced ion motion in an inserted polyelectrolyte layer of an OFET structure can potentially improve sensor technologies beyond humidity-responsive systems.

Counterion-induced reversibly switchable transparency in smart windows.

Smart windows that can reversibly alternate between extreme optical characteristics via clicking counteranions of different hydration energies were developed on glass substrates through the facile spray-casting of poly[2-(methacryloyloxy)ethyltrimethylammonium chloride-co-3-(trimethoxysilyl)propyl methacrylate]. The optical transmittance was either 90.9% or 0% over the whole spectral range when alternately immersed in solutions containing thiocyanate (SCN(-)) or bis(trifluoromethane)sulfonimide (TFSI(-)) ions, respectively. The extreme optical transitions were attributed to formation of microporous structures via the molecular aggregation of polyelectrolyte chains bearing TFSI(-) ions in methanol. Because the smart windows were either highly transparent toward or completely blocking of incident light upon direct counterion exchange, this kind of nanotechnology may provide a new platform for efficiently conserving on energy usage in the interior of buildings.

Interpenetrating polymer network dielectrics for high-performance organic field-effect transistors

We have demonstrated the preparation of interpenetrating polymer network (IPN) dielectrics for use in high-performance organic field-effect transistors by blending commercially available polymers (PMMA, PtBMA, and PS) with the crosslinkable polymeric silsesquiazane (SSQZ). This facile blending method is a powerful means of enhancing the electrical strength of polymer dielectrics due to the formation of a siloxane network structure interspersed among the polymer chains. We found that the leakage currents for the PMMA and PtBMA gate dielectrics blended with SSQZ significantly decreased, by as much as two orders of magnitude, compared with the pristine cases. These remarkable enhancements in the dielectric properties arose from decreases in the free volume and in the thermal dynamic motions of the polymer chains due to formation of the polysiloxane network. The IPN gate dielectrics provide a facile method for using commercially available polymers to fabricate polymer gate dielectrics with strong electrical strengths.

Electrospun smart fabrics that display pH-responsive tunable wettability

Smart fabrics that reversibly switch between superhydrophobicity and superhydrophilicity were developed using a combination of hierarchical nanostructured fibrous webs and a pH-responsive polymer that experiences pH-dependent conformational transitions, poly[2-(diisopropylamino)ethylmethacrylate-co-3-(trimethoxysilyl)propylmethacrylate].

Counterions-exchangeable, multifunctional polyelectrolyte fabrics

We demonstrated multifunctional electrospun polyelectrolyte fabrics with switchable superhydrophobicity as well as oleophobicity. To produce this smart fabric, we used a strategy that combines electrospinning to fabricate nanofibrous templates with nanopores and a simple coating of the polyelectrolyte that can exchange counterions with various hydration energies. The ion exchange of polyelectrolyte embedded in nanoporous fibrous webs leads to switchable wetting behavior in water and oil. This electrospun fabric was also utilized as chemical filters for the efficient removal of sulfur dioxide (SO2) from waste gas streams.